I've had a Fenix 3 and FR935, and regularly made use of a hotkey to go back to the watch face during an activity. For instance, I'm biking and want to look at the compass: I click the watch face hotkey, then scroll to the right widget/glance, then click "start" again to go back to the activity.
I noticed today on my 955 that, even though the timer keeps running, GPS stops updating when I'm viewing the watch face. This is NOT true when other non-activity screens are up, for instance if I use a hotkey to pull up the stopwatch and use that during an activity. I can look at the stopwatch screen the entire activity, and distance and time will accrue normally.
This could be related to another bug I've noticed: if I go back to the watch face, then return to i.e. a running activity, CIQ fields in that running activity crash. Specifically, I use the "Single Field" field for both running and cycling. If I go back to the watch face during an activity, I just see "CIQ!" on this screen once I go back to the activity. I can recover it by editing the screen and re-selecting "Single Field," without ever stopping the activity. Again, this doesn't happen if I simply bring up a stopwatch/timer - only if I go all the way back to the watch face.
